Thats a good point sherwood. I've been making some nucs from various materials to see what I prefer. I've tried 9mm good ply, 9mm osb, 18mm cheap ply, 19mm cedar, and polystyrene.
Polystyrene seems good for overwintering a small colony of slow bees but it's neither cheap nor durable.
I can buy cedar flatpack nuc hive with floor, roof with metal, cover board and brood box for £30 in seconds grade, which I think is possibly the best all round solution.
However making your own is cheaper and for that I think 18mm cheap ply is best. You'll get at least two out of a sheet, or five from two sheets, which works out about £10 each nuc hive.
I have learned its useful to make them exactly the same size as a full size hive but only half the width. This alows you to put two nuc bbs on top of one normal bb and allow the bees to expand up into them, with one normal roof and cover board on.
Using the above philosophy, and national dimensions, this means they are 460mm (some say 461) x 230mm, and whatever height suits your frames (mine are 14x12) so 315mm seems to fit (separate floor with 20mm bee space).
So if you make them from 18mm ply the internal width is 194mm (Er, ... or as pete said 195mm).
BTW pete, do you use Sadolin quick-dry, or Sadolin Extra ?
JC.
p.s. atm I'm only in my 1st full year of beeking so I'm stronger on thinking about it than actually doing it successfully, but I've got four small colonies going now in 14x12 nats and am hoping to split them to eight and over-winter four as nucs. If I get that far I've an idea to make polythene bags full of poly beads to lag the outside of the nucs in case it gets v cold.